there is no permanently visible ruler in RWPaint, but current coordinates of the mouse are displayed in the status bar. If you do not want to subtract the coordinates of the start and end positions in your head, you can for example switch to the transform tool and then drag a rectangle around the measured object - you'll then see the dimensions in pixels in the status bar to the left of the current coordinates. The coordinates and dimensions are always in pixels. You can set the resolution when creating new images, but some file formats do not support it and so it is best to specify the physical dimensions of an image just before printing. If you use the Print plug-in, you can specify the width or height of the printed image in inches or millimeters in the Print window.

You can create animated GIF, PNG and WebP images with RWPaint. You'll probably want to stick with GIF, because animated PNGs are only supported in Firefox and animated WebP images are not yet supported in any browser, but they probably will be supported in future Chrome builds.